Canisters for controlling evaporative emissions from vehicles are well known. Such emissions are created at two particular times: first, while a vehicle is being refueled, and vapor-laden air is being displaced from the fuel tank (known in the art as “refueling emissions”); and second, while a vehicle is shut down for an extended period, and fuel-laden adsorber in a canister spontaneously degases to the atmosphere (known in the art as “diurnal emissions” or “bleed” emissions).
In the prior art, refueling emissions are collected typically by a canister disposed between a port in the vehicle fuel tank and outside atmosphere. The canister has two side-by-side chambers filled with an adsorptive carbon composition and connected at one end such that gases follow a U-shaped flow path through the canister. Valving is provided such that the canister can be degassed of fuel by engine vacuum when the vehicle's engine is restarted. In desorption mode, outside air is drawn into the canister in reverse flow through the adsorption mode exhaust port and sweeps adsorbed fuel from the carbon beds into the engine intake manifold.
The California Air Resources Board (CARB) has published more stringent emissions regulations, known generally as Low Emission Vehicle II (LEV II) and Partial Zero Emission Vehicle (PZEV). PZEV is more stringent than LEV II.
The CARB first adopted LEV standards in 1990. These first LEV standards ran from 1994 through 2003. LEV II regulations, running from 2004 through 2010, represent continuing progress in emission reductions. As the state's passenger vehicle fleet continues to grow and more sport utility vehicles and pickup trucks are used as passenger cars rather than work vehicles, the new, more stringent LEV II standards are necessary for California to meet federally-mandated clean air goals outlined in the 1994 State Implementation Plan (SIP). When LEV II is fully implemented in 2010, it is estimated that smog-forming emissions in the Los Angeles area will be reduced by 57 tons per day, while the statewide reduction will be 155 tons per day.
PZEV-conforming vehicles are those that have achieved the CARB's cleanest tailpipe emission standard—the Super Ultra Low Emission Vehicle (SULEV) standard. In addition, they have nearly zero evaporative emissions and their emission control equipment is warranted for 15 years/150,000 miles.
Prior art canisters as described above have been capable of meeting the original LEV standards, and with the addition of a downstream carbon “scrubber” can meet the PZEV standards. For both LEV II and PZEV applications, the canister's diurnal emission performance can be greatly improved by increased flow path length and partitioning of the carbon bed. These features allow the carbon closest to the fresh air source to be very well purged, and keep migrating hydrocarbon vapors away from the atmospheric port.
One means known in the art for partitioning a canister is to provide a horizontal plate in the carbon bed, breaking the bed into two shorter chambers. An opening in the plate allows flow between chambers. The opening must be large enough to allow for acceptable flow restriction performance. Because the driving pressure for flow through a canister is very low, it is an important design consideration that flow restriction be kept to a minimum. This configuration requires two separate filings and settlings of loose carbon into the canister and thus increases manufacturing cost.
A downstream carbon scrubber to meet PZEV diurnal emission levels is known to be installed either in line at the atmospheric port of a canister or in an added dedicated chamber molded onto the canister housing itself. Either configuration increases the overall size of a canister, which is undesirable because of space considerations in the region of a vehicle wherein a canister is installed. Thus what is needed in the art is means for incorporating a scrubber within the existing volume of a prior art canister while still meeting PZEV emission standards.
Another means for improving the efficiency of a canister is to increase the L/D ratio wherein L is the length of the flow path and D is its average diameter. Therefore, what is further needed in the art is an improved canister having an increased L/D ratio.
It is a principal object of the present invention to provide improved emissions adsorption means for meeting both the LEV II and PZEV standards at lower manufacturing cost, greater simplicity of assembly, and low footprint in a vehicle.